1. Field of the Invention
The present invention relates to a two-way valve for switching a valve-open state and a valve-closed state of a valve plug which is displaceable, for example, by a pilot pressure to switch the flowing of a fluid.
2. Description of the Related Art
A two-way valve device is used to switch a communication state and a non-communication state of a fluid flowing through a fluid passage. In the device, a valve is displaceable by the pilot pressure to switch the flowing state of the fluid.
As shown in FIG. 7, for example, such a two-way valve 1 comprises a housing 3 formed with a fluid passage 2c which communicates with an inlet port 2a and an outlet port 2b for supplying/discharging the fluid. A bonnet 5, which includes an upper piston 4a and a lower piston 4b provided displaceably therein, is connected to an upper portion of the housing 3. A spring 7 is interposed between the upper piston 4a and a cap member 6 which is installed in the bonnet 5. A diaphragm 9 is connected to a lower portion of a shaft 8 which is connected to the upper piston 4a and the lower piston 4b. 
The diaphragm 9 is formed of a resin material. A circumferential edge section 11 of a thin-walled film section 10, which extends radially outwardly from a substantially central portion, is interposed between the housing 3 and a guide member 12 which is provided in the housing 3.
The upper piston 4a and the lower piston 4b are displaced upwardly in the axial direction integrally by the pilot pressure introduced into the space disposed under the lower piston 4b. Accordingly, the shaft 8 integrally connected to the upper piston 4a and the lower piston 4b is displaced upwardly. Thus, the diaphragm 9 is separated from a valve seat 13 provided on the housing 3, which is a valve-open state.
When the supply of the pilot pressure is stopped to be a state open to the atmospheric air, the upper piston 4a is pressed downwardly by the spring force of the spring 7. The diaphragm 9 is seated on the valve seat 13 by the shaft 8 when the upper piston 4a is displaced. Accordingly, the flow of the fluid flowing through the fluid passage 2c is blocked (see, for example, columns 2 and 3 of U.S. Pat. No. 5,002,086).
In the two-way valve 1 described in U.S. Pat. No. 5,002,086, when some gas or liquid flows through the fluid passage 2c of the housing 3, the gas itself or the gas generated by the vaporization of the liquid may enter the interior of the two-way valve 1 as a result of the permeation through the thin-walled film section 10 of the diaphragm 9 of the resin material, depending on the type of the gas or the liquid (for example, hydrochloric acid).
In such a situation, the gas may enter the interior of the bonnet 5, and the durability of the spring 7 provided in the bonnet 5 may be deteriorated by the influence of the gas.